(a) Technical Field
The present disclosure relates to a method for preparing lactide, particularly by reacting lactic acid oligomers in the presence of an ionic solvent.
(b) Background Art
The amazing industrialization since the 20th century appears to be largely based on fossil fuel resources, particularly petroleum. With the rapid industrial development and population growth, the petroleum consumption has been increased continuously as well. Petroleum is an unrenewable resource with a limited amount of reserves that will soon be exhausted. Recently, it has been found that the carbon dioxide generated by fossil fuel consumption is one of the main causes of global warming. As such, researchers are striving to improve fuel efficiency so as to reduce carbon dioxide emissions and to reduce dependence on petroleum.
Polymers derived from plants, i.e., biomass polymers, can be prepared by a chemical or biological process from renewable plant resources such as corn, bean, sugar cane, wood, etc. A value of biomass polymers lies in their potential to solve environmental problems through carbon dioxide reduction rather than in biodegradability. Among biomass polymers, polylactic acid is a carbon neutral, environment-friendly, thermoplastic, linear aliphatic polyester, It is derived from corn starch or potato starch through fermentation or it is prepared by polymerizing sugar monomers obtained from saccharification of plant-derived cellulose followed by fermentation.
Despite the various advantages of polylactic acid, however, it does not appear to be suitable for use in automobile parts, because of its low impact resistance, low heat deflection temperature, etc., as compared to the petroleum-based chemical polymers. In particular, polylactic acid has low impact strength due to its brittleness, which thus delimits its application in automobile parts.
For this reason, industrial application of polylactic acid resin is limited due to its inferior physical properties when compared to the general-use polymer materials. In particular, for use in automobile engine and chassis parts requiring high heat resistance and impact resistance, improvement of the physical properties of polylactic acid resin is essential. As a strategy to solve this problem, a technique of preparing a stereo-complex resin by blending the optical isomers of polylactic acid is often used.
Conventionally, a method for manufacturing lactide is composed of polymerizing lactic acid into a low molecular weight oligomer and depolymerizing the low molecular weight oligomer into lactide. In the first step, the oligomer is polymerized under the conditions of 150° C. and 0.3 atm, and moisture generated during this step is removed. In the second step, the synthesis of lactide is carried out in the presence of a catalyst, wherein the catalyst may include tin powder, tin halides or tin carboxylates (EP Patent Nos. 261,572 and 275,581); tin alkoxides (United Kingdom Patent No. 1,007,347); and zinc or tin (EP Patent No 264,926 and U.S. Pat. No. 4,797,468).
In this conventional method, since the polymerization and depolymerization occur simultaneously in the second step of preparing lactides, the molecular weight of lactide obtained thereby is gradually increased. Thus, in order to improve the reaction yield by inducing the synthetic reaction continuously, there is a need to increase a reaction temperature continually. In particular, the synthesis of lactide initiates under the conditions of 170° C. and 0.1 atm in the presence of LiCO as a catalyst. However, the reaction temperature should be increased gradually up to 200° C. or higher so as to increase the reaction yield. It was reported that a yield above 90% was obtained through the reaction at 200° C. for 2 hours based on a 500 cc reactor. Since the synthesis of lactide is carried out at a high reaction temperature, there is a need to wash a reactor by using a solvent, and thus, there is a problem in generating a large quantity of waste liquid caused by the solvent. Further, since the washing step is indispensable, it is very difficult to utilize continuous processing in this conventional method.
As one conventional method for preparing lactide, Korean Patent No. 171,432 describes a method for preparing lactide by treating a aqueous lactic acid feed so as to remove water therefrom, terminating the treatment to produce a crude lactide product, and separating lactides from the crude lactide product.
Japanese Patent Application Publication No. 2004-0149419 describes a method for preparing lactide, which is characterized by azeotropic dehydration of a mixture including water-containing crude lactide and an azeotropic solvent, precipitation of lactide by using the solvent after the azeotropic dehydration, solid/liquid separation of the solvent into a solid phase and an azeotropic solvent phase, and collection of lactide from the solid phase.
Japanese Patent Application Publication No. 1999-0209370 describes a method for preparing lactide, which includes the steps of: synthesizing a composition including polylactic acid alone or polylactic acid and lactide through dealcoholization by heating lactic acid ester in the presence of a monobutyltin compound, and synthesizing lactide by heating the composition polylactic acid alone or polylactic acid and lactide obtained in the above step in the presence of a monobutyltin compound.
Korean Patent Application Publication No. 2010-0005820 describes a method for preparing optically pure lactide, including: (A) dissolving lactic acid in an organic solvent; (B) adding an enzyme to the organic solvent solution of lactic acid and stirring the resulting mixture; and (C) separating optically pure lactide from the resulting mixture.
However, none of the preparation methods describes in the above patents can overcome the problems of the prior art as mentioned above.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.